Influence of Anthocyanin Expression on the Performance of Photosynthesis in Sweet Orange, Citrus sinensis (L.) Osbeck.

Anthocyanins are a class of natural pigments that accumulate transiently or permanently in plant tissues, often in response to abiotic and biotic stresses. They play a photoprotective role by attenuating the irradiance incident on the photochemical apparatus and quenching oxyradicals through their powerful anti-oxidative function. The objective of the current study is to understand the impact of introducing Vitis vinifera mybA1 (VvmybA1) in 'Hamlin' sweet orange trees on various aspects, including photosynthetic performance, pigment composition, and gene expression related to photosynthesis and light harvesting. We describe the relationship between anthocyanin accumulation and photosynthetic measurements in genetically modified 'Hamlin' sweet orange trees expressing the grapevine-derived Vitis vinifera mybA1 (VvmybA1). The juvenile leaves of transgenic plants displayed an intense purple color compared to the mature leaves, and microscopic visualization showed anthocyanin accumulation primarily in the leaf epidermal cells. Under optimal growth conditions, there were no significant differences in leaf gas exchange variables, suggesting normal photosynthetic performance. The chlorophyll fluorescence maximum quantum yield of PSII was slightly reduced in VvmybA1 transgenic leaves compared to the performance of the control leaves, while the total performance index per absorbance remained unaffected. Comparison of the chlorophyll and carotenoid pigment contents revealed that chlorophyllide a and carotenoid pigments, including trans-neoxanthin, trans-violaxanthin, cis-violaxanthin, zeaxanthin, antheraxanthin, and total xanthophylls were enhanced in VvmybA1 transgenic leaves. Although there were no significant changes in the rates of the gas exchange parameters, we recorded a high relative expression of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (RuBP) and rubisco activase (RCA) in the mature leaves of transgenic plants, indicating activation of Rubisco. Our findings confirm an efficient photoacclimation of the photosynthetic apparatus, allowing the transgenic line to maintain a photosynthetic performance similar to that of the wild type.

Chilling and forcing temperatures interact to predict the onset of wood formation in Northern Hemisphere conifers.

The phenology of wood formation is a critical process to consider for predicting how trees from the temperate and boreal zones may react to climate change. Compared to leaf phenology, however, the determinism of wood phenology is still poorly known. Here, we compared for the first time three alternative ecophysiological model classes (threshold models, heat-sum models and chilling-influenced heat-sum models) and an empirical model in their ability to predict the starting date of xylem cell enlargement in spring, for four major Northern Hemisphere conifers (Larix decidua, Pinus sylvestris, Picea abies and Picea mariana). We fitted models with Bayesian inference to wood phenological data collected for 220 site-years over Europe and Canada. The chilling-influenced heat-sum model received most support for all the four studied species, predicting validation data with a 7.7-day error, which is within one day of the observed data resolution. We conclude that both chilling and forcing temperatures determine the onset of wood formation in Northern Hemisphere conifers. Importantly, the chilling-influenced heat-sum model showed virtually no spatial bias whichever the species, despite the large environmental gradients considered. This suggests that the spring onset of wood formation is far less affected by local adaptation than by environmentally driven plasticity. In a context of climate change, we therefore expect rising winter-spring temperature to exert ambivalent effects on the spring onset of wood formation, tending to hasten it through the accumulation of forcing temperature, but imposing a higher forcing temperature requirement through the lower accumulation of chilling.

Climate change impacts on tree ranges: model intercomparison facilitates understanding and quantification of uncertainty.

Model-based projections of shifts in tree species range due to climate change are becoming an important decision support tool for forest management. However, poorly evaluated sources of uncertainty require more scrutiny before relying heavily on models for decision-making. We evaluated uncertainty arising from differences in model formulations of tree response to climate change based on a rigorous intercomparison of projections of tree distributions in France. We compared eight models ranging from niche-based to process-based models. On average, models project large range contractions of temperate tree species in lowlands due to climate change. There was substantial disagreement between models for temperate broadleaf deciduous tree species, but differences in the capacity of models to account for rising CO(2) impacts explained much of the disagreement. There was good quantitative agreement among models concerning the range contractions for Scots pine. For the dominant Mediterranean tree species, Holm oak, all models foresee substantial range expansion.